Dear Andrei, I did 2x2x2 supecell block and i get an increased bandgap(1.9 eV) almost twice to bulk(0.9 eV). I did 4x4x4 , results are much better although still a bit off. Although i did not put the supercellblock back into siesta with twice the parameters(will do it). Now , i really start scratching my head. I checked manually bulk cell and it is R point for sure (1/2,1/2,1/2). Any ideas ?
Thank you. On Wed, Aug 21, 2013 at 8:18 AM, <[email protected]> wrote: > OK Andrei. > Now the chemical formula makes sense. > (Is the methyl ammonium rotating in its site? > Disordered in this sense? - Must be funny... - A nice task for Siesta) > > 1. The 2x2x2 supercell would nicely map X, M and R points > of the primitive cell onto its new Gamma. Hopefully it's easy > to trace "by hand", checking contributions from different atoms > to the wavefunction at these points in case of difficulty. > I don't understand why do you need a 3x3x3 supercell if you are > interested just in folding. Must be a long calculation... > > 2. Be careful with the supercell block; my suggestion is > to use it just for generating the supercell, then put the generated > coordinates into the input (or start from XV) and deactivate > the supercell block. Otherwise it is easy to make an error. > > Good luck > > Andrei Postnikov > > > > Thank you Andrei. > > > > 1. I do apologize for the typo. System is pervoskite (CH3NH3)SnI3. It has > > 12 atoms in cubic cell. > > 2. Going to supercell should not increase the bandgap. I agree on this. > > 3. I did tetragonal cell of the same system of 48 atoms , and it looks ok > > if i use superecell but it has bandgap at G-point. > > 4. I looked structures before, basically 12 atoms at 8x8x8 k-mesh gives > > bandgap of 0.95 eV, whereas with %blocksupercell 3 3 3 %endblcoksupercell > > with only G point gives 1.6 eV. > > 5. And you are right, i also started to suspect that zone folding is the > > problem(i mean, my whole point of doing supercell calculations is to map > > R-point into G-point). > > 6. Visually it looks ok. It contains methylammonium. The only thing it > > might have is the dipole moment. > > > > Witll try to use 2x2x2,4x4x4 (It should correctly map R point onto G > > point, > > correct me if i'm wrong ) supercell to see whether problem disappears. > > > > > > With Best Regards, Andrei Buin. > > > > > > > > On Wed, Aug 21, 2013 at 6:29 AM, <[email protected]> wrote: > > > >> Dear Andrei: > >> I don't see how your system can be a perovskite, > >> nor how CH3N3SnI3 sums up to 12 atoms. I'll appreciate if you > >> give a hint, just for my education. > >> However, this is beyond the point. > >> In principle, a mere passing to a supercell > >> should not result in an increase of the band gap. > >> The mapping of k-points may be not exact, i.e., > >> Gamma of the 3*3*3 supercell maps onto 0, 1/3, 2/3 [*(2*pi/a)] > >> divisions along axes of the reciprocal cell, > >> none of which points includes R=(1/2 1/2 1/2). You can check > >> what the band gap (in the primitive cell) is at (1/3 1/3 1/3), > >> or at (3/8 3/8 3/8), the closest of your k-points at the 8x8x8 mesh. > >> [However, you say that you have an automatic k sampling, then > >> the chances are that your mesh is shifted... and does not contain > >> the R point..?] > >> Apart from these tests, I think that some simple error in structure > >> (as simple as a wrong lattice constant in a supercell, sorry) > >> is a very good candidate for this kind of problem. > >> To check it, it might be a good idea to have a view at both structures > >> (the primitive one and the supercell) as stored in corresponding XV > >> files, > >> and to compare their corresponding DOS. > >> > >> Again apart from all this, if your system contains methyl azide, > >> I think it could be tricky to initialize the different charge states > >> at different N atoms correctly... If you just let it go as it wish > >> the calculation may converge to something wrong... Even to differently > >> wrong in two different supercells. > >> > >> Sorry for too many suggestions. > >> > >> Best regards > >> > >> Andrei Postnikov > >> > >> > >> > Dear Siesta users, > >> > > >> > I'm having very bizarre problem. > >> > I'm simulating the primitive cubic unit cell(12 atoms) of the > >> CH3N3SnI3 > >> > perovksite with GGA and PBE and gives me band gap of Eg=0.9 eV at R > >> point > >> > with 8x8x8 Moshkrof automatic k sampling(i know it's wrong in DFT, but > >> > still) and then i use supercell approach built in with 3x3x3 unit > >> > cell(432 atoms) with Gamma point only and i get a bangap of 1.6 eV. > >> I've > >> > checked against plane cutoff convergence seems ok. I thought maybe > >> > dispersion is too big so one should sample it more finely, but it > >> turns > >> > out meff=0.13me, which should be fine. Any ideas ? > >> > > >> > > >> > With Best Regards, Andrei Buin. > >> > > >> > >> > > > >
